1. Field of the Invention
The present invention relates to a method of manufacturing a molding material in which a large number of metal fibers are dispersed in a resin material and an apparatus therefore.
2. Description of the Related Art
In order to prevent leakage of or invasion of electromagnetic waves, electronic or electric devices have a casing which is made of a conductive resin material capable of shielding them against electromagnetic waves.
Such a casing is generally manufactured by molding master pellets which are a molding material containing metal fiber or a mixture of those master pellets and natural pellets of the same resin using an adequate molding machine or a die.
Various methods of manufacturing the master pellets containing metal fibers are known. In one of them, metal fibers and natural pellets are extruded from an extruder while being heated and mixed with each other, the extruded articles being cut into an adequate length. In another method, a resin is coated on the periphery of a bundle of metal fibers using an extruder, the extruded articles being cut into an adequate length (proposed in Japanese Patent Laid-Open No. 18315/1985). In still another method, metal fibers are dipped in a resin solution so as to impregnate the metal fibers with the resin, and the same resin is then coated on the periphery of the metal fibers after they have been dried using an extruder, the extruded articles being cut into an adequate length (disclosed in Japanese Patent Publication No. 26783/1988).
However, in the manufacturing method in which the metal fibers and the natural pellets are heated and mixed, since it is very difficult for the metal fibers to be uniformly dispersed in the natural pellets during the mixture, variations in the density of fibers contained in the extruded articles easily occur. Also, since long metal fibers cannot be used due to prevention of bending of the metal fibers and intertwinement of the bending metal fibers, the resultant extruded articles cannot be made sufficiently conductive.
In the manufacturing method in which a resin is coated on the periphery of the bundle of metal fibers, since the bundle of metal fibers is not easily impregnated with the resin, cracking of the coated resin or removal of the metal fibers easily occur during the cutting operation. These make manufacture of master pellets which are satisfactory as a molding material impossible.
In the manufacturing method in which the bundle of metal fibers is impregnated with resin, the amount of resin impregnated differs greatly depending on the density at which the bundle of metal fibers is converged. In consequence, manufacture of master pellets containing metal fibers at a predetermined rate is difficult, and production cost is increased due to a large number of manhours.